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Observations about Falling Balls
When you drop a ball, it
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begins at rest,
soon acquires a considerable downward speed,
and covers more and more distance each second
When you tossed a ball straight up, it
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rises to a certain height,
comes momentarily to a stop,
and then descends, much like a dropped ball
A thrown ball travels in an arc
Turn off all electronic devices
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6 Questions about Falling Balls
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Why does a dropped ball fall downward?
How differently do different balls fall?
How would a ball fall on the moon?
How does a falling ball move after it is dropped?
How can a ball move upward and still be falling?
How does a ball's horizontal motion affect its fall?
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Question 1
Q: Why does a dropped ball fall downward?
A: The ball’s downward weight causes it to accelerate downward
Earth’s gravity exerts a downward force on the ball
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That force on the ball due to Earth’s gravity is called the ball’s weight
The ball’s weight points toward earth’s center (it defines downward)
A falling ball experiences only one force: its weight
A falling ball’s weight causes it to accelerate downward
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Question 2
Q: How differently do different balls fall?
A: Not differently. They all fall together!
A ball’s weight is proportional to its mass
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If you divided a ball’s weight by its mass, you always get the same value:
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Near Earth’s surface, every kilogram of mass weighs 9.8 newtons!
Acceleration Due to Gravity
According to Newton’s second law,
The only force acting on a falling ball is its weight,
That ratio is the acceleration of any falling object near Earth’s surface!
It is called the acceleration due to gravity
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Question 3
Question 4
Q: How would a ball fall on the moon?
A: It would fall more slowly.
Q: How does a falling ball move after it is dropped?
A: It accelerates downward, covering more distance each second
Gravity depends on mass of planet/moon and distance from its center
A falling ball experiences only its weight
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Moon’s mass is small, but its radius is also small, so
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Its acceleration is constant and downward
Its velocity increases in the downward direction
When dropped from rest,
Earth’s gravity actually varies slightly with location
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You weigh slightly less at the Equator than at the North or South Pole
You weigh very slightly less on a mountaintop than in a valley
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the ball’s velocity starts at zero and
increases in the downward direction
the ball’s altitude decreases at
an ever faster rate
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Question 5
Question 6
Q: How can a ball move upward and still be falling?
A: It may be moving upward, but it is still accelerating downward!
Q: How does a ball's horizontal motion affect its fall?
A: It doesn’t. The ball falls vertically, but coasts horizontally.
A falling ball accelerates downward, but its
initial velocity can be anything, even upward!
When thrown upward,
Ball’s acceleration is purely
vertical (downward)
It falls vertically
It coasts horizontally
Its path is a parabolic arc
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ball’s velocity starts upward but
increases downward
ball’s altitude increases at
an ever slower rate until…
velocity is momentarily zero
and then ball falls downward…
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Summary About Falling Balls
Without gravity, an isolated ball would coast
With gravity, an isolated ball
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experiences its weight,
accelerates downward,
and its velocity becomes increasingly downward
Whether going up or down, it’s still falling
It can coast horizontally while falling vertically
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